High strength and high density intraluminal wire stent

ABSTRACT

An intraluminally implantable stent is formed of helically wound wire. The stent has a generally elongate tubular configuration and is radially expandable after implantation in a body vessel. The wire includes successively formed waves along the length of the wire. When helically wound into a tube, the waves are longitudinally nested along the longitudinal extent of the stent so as to form a densely compacted wire configuration. After radial expansion the stent maintains high radial compressive strength and wire density to retard tissue ingrowth.

FIELD OF THE INVENTION

[0001] The present invention relates generally to implantableintraluminal stents and more particularly, the present invention relatesto an improved high strength intraluminal stent having increased wiredensity.

BACKGROUND OF THE INVENTION:

[0002] It is well known to employ endoprostheses for the treatment ofdiseases of various body vessels. Intraluminal devices of this type arecommonly referred to as stents. These devices are typicallyintraluminally implanted by use of a catheter into various body organssuch as the vascular system, the bile tract and the urogenital tract.Many of the stents are radially compressible and expandable so that theymay be easily inserted through the lumen in a collapsed or unexpandedstate. Some stent designs are generally flexible so they can be easilymaneuvered through the various body vessels for deployment. Once inposition, the stent may be deployed by allowing the stent to expand toits uncompressed state or by expanding the stent by use of a catheterballoon.

[0003] As stents are normally employed to hold open an otherwiseblocked, constricted or occluded lumen; a stent must exhibit arelatively high degree of radial or hoop strength in its expanded state.The need for such high strength stents is especially seen in stents usedin the urogenital or bile tracts where disease or growth adjacent thelumen may exert an external compressive force thereon which would tendto close the lumen.

[0004] One particular form of stent currently being used is a wirestent. Stents of this type are formed by single or multiple strands ofwire which may be formed into a shape such as a mesh coil, helix or thelike which is flexible and readily expandable. The spaces between thecoiled wire permit such flexibility and expansion. However, in certainsituations, such as when the stent is employed in the urogenital or biletract, it is also desirable to inhibit tissue ingrowth through thestent. Such ingrowth through the stent could have a tendency to recloseor occlude the open lumen. The open spaces between the wires forming thestent, while facilitating flexibility and expansion, have a tendency toallow such undesirable tissue ingrowth.

[0005] Attempts have been made to provide a stent which has less openspace and more solid wire. U.S. Pat. No. 5,133,732 shows a wire stentwhere the wire forming the stent is overlapped during formation toprovide less open space. However such overlapping wire increases thediameter of the stent and has a tendency to reduce flexibility and makeimplantation more difficult. It is therefore desirable to provide a wirestent which exhibits high compressive strength and full flexibilitywithout allowing extensive ingrowth therethrough.

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide anintraluminal stent which exhibits high compressive strength and isresistive to tissue ingrowth.

[0007] It is a further object of the present invention to provide aflexible wire stent having high compressive strength and maximum wiredensity to inhibit tissue ingrowth.

[0008] In the efficient attainment of these and other objects, thepresent invention provides an intraluminal stent including a generallyelongate tubular body formed of a wound wire. The wire forming the stentis formed into successively shaped waves, the waves being helicallywound along the length of the tube. The longitudinal spacing between thehelical windings of the tube is formed to be less than twice theamplitude of the waves thereby resulting in a dense wire configuration.

[0009] As more particularly shown by way of the preferred embodimentherein, an intraluminal wire stent includes longitudinally adjacentwaves being nested along the length of the tubular body. The peaks orapices of the longitudinally nested waves are linerally aligned.Further, the intraluminal stent so constructed would have a percentageof open surface area in relationship to the total surface area of thestent which is less than 30% in the closed state, resulting in less openarea upon expansion which would inhibit tissue ingrowth.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a perspective view of a conventional helical coil formedof a single wound wire.

[0011]FIG. 2 is a perspective view of the stent of the presentinvention.

[0012]FIG. 3 is a perspective view of the stent of FIG. 1 exhibitinglongitudinal flexibility.

[0013]FIG. 4 is a schematic showing of one wave of the wire forming thestent of FIG. 2.

[0014]FIG. 5 is a schematic showing of nested longitudinally adjacentwaves of the stent of FIG. 2.

[0015]FIG. 6 is a perspective view of the stent of FIG. 2 shown in theopen or exposed condition.

[0016]FIG. 7 shows a portion of a further embodiment of a wire used toform a stent in accordance with the present invention.

[0017]FIG. 8 shows a still further embodiment of a wire used to form astent of the present invention, partially wound around a formingmandrel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:

[0018] A simple helically formed coil spring 10 is shown in FIG. 1. Coilspring 10 is formed of a single metallic wire 12 which for stentpurposes may be formed of a suitably flexible biocompatible metal. Thewire coil spring 10 defines generally a cylindrical tubular shape whichis radially expandable upon application of outward radial pressure fromthe interior thereof.

[0019] The present invention shown in FIG. 2, improves upon the simplecoil spring 10 shown in FIG. 1. However with reference to FIG. 1,certain terminology used hereinthroughout may be defined. As mentioned,the spring defines a generally elongate cylindrically tubular shapelying along a central axis χ. Wire 12 is helically wound, for exampleagainst a constant diameter mandrel (not shown), to form alongitudinally extending structure consisting of wire 12 and spaces orpitch 16 therebetween. Each individual winding 14 may be defined as thewire segment traversing one complete revolution around axis χ. As thewire is helically coiled about axis χ, each winding is successivelylongitudinally spaced from the next adjacent winding by a givendistance.

[0020] For present purposes, the axial spacing between any point on thewire coil spring 10 to the point defining the next successive windingmay be thought of as the pitch 16 of the wire coil spring 10. As sodefined, the pitch of the coil spring 10 defines the spacing betweenwindings and therefore the degree of compactness or compression of thewire coil spring 10.

[0021] Also with reference to FIG. 1, as the wire coil spring 10 has agenerally cylindrical tubular shape, it defines an outside diameter d₁and an inside diameter d₂ which would typically differ by twice thediameter d₃ of wire 12. Further, wire coil spring 10 generally definesan outer cylindrical surface area along its length which may be thoughtof as being composed of solid surface portions defined by the outwardfacing surface of wire 12 itself and open surface portions defined bythe spaces or pitch 16 multiplied by the number of wire windings 14. Theratio of open surface space to solid surface space may be varied byvarying the so-defined pitch 16 of the wire coil spring 10. A smallerpitch coil, where the windings are more compacted or compressed, wouldresult in an outer surface area having less open space than a coilformed to have greater spacing or pitch between the wire windings.

[0022] Having set forth the definitional convention usedhereinthroughout, the present invention may be described with referencespecifically to FIGS. 2-6. A wire stent 20 of the present invention isshown in FIG. 2. Wire stent 20 is generally in the form of an elongatecylindrically shaped tubular member defining a central open passage 21therethrough. Stent 20 is formed of multiple windings 24 of a singlewire 22 which in the present invention is metallic, preferably tantalum,as such wire exhibits sufficient spring elasticity for purposes whichwill be described in further detail hereinbelow.

[0023] While stent 20 may be formed by helically winding wire 22 much ina manner shown with respect to FIG. 1 to form wire coil spring 10, thepresent invention contemplates preshaping the wire 22 itself along itslength prior to helically coiling the wire.

[0024] Referring now to FIG. 4, wire 22 in an elongate pre-helicallycoiled configuration may be shaped in a manner having a longitudinallyextending wave-like pattern. Wave pattern 25 is defined by a pluralityof continuously repeating wave lengths 27 therealong. It has been foundadvantageously that the waves may take the form specifically shown inFIGS. 4 and 5 for optimum results as a wire stent. However, forexplanation purposes, the wave-like pattern 25 generally functionsmathematically as sinusoidal wave, having a given amplitude A asmeasured from a central axis y and a peak-to-peak amplitude of 2A. Thewave pattern 25 has a uniform preselected period λ equal to thetransverse extent of a single wave length. The geometry of each wavelength 27 is shown in FIG. 4.

[0025] The wave-like configuration imparted to wire 22 may beaccomplished in a variety of forming techniques. One such technique isto pass wire 22 between the teeth of intermeshed gears (not shown) whichwould place a generally uniform sinusoidal wave-like crimp along thelength of the wire. Other techniques may be used to form the specificshape shown in FIG. 4. Wire 22 may be passed through a pair of gear-likeoverlapping wheels (not shown) having depending interdigitating pins. Byarranging the size, position and spacing of the pins, various wave-likeconfigurations may be achieved. The particular shape shown withreference to FIGS. 4 and 5 has been selected as each wave length 27includes a pair of non-curved linear sections 29 between curved peaks31. As will be described with respect to FIG. 5, this configurationallows the waves to be stacked or nested with maximum compactness whenthe wire is helically wound around a forming mandrel (FIG. 8) into theshape shown in FIG. 2.

[0026] Referring now to FIG. 5, schematically shown is a portion ofstent 20 of FIG. 2 which has been cut once, parallel to the χ axis andflattened after being wound in a helical fashion such as that describedwith respect to the wire coil spring 10 of FIG. 1. Wire 22 formed in themanner shown and described with respect to FIG. 4, may be helicallywound around an appropriately shaped mandrel (FIG. 8). The width of themandrel is selected in combination with the frequency and period of thewaves forming wire 22 so that upon helical coiling therearound the wavesforming each winding 24 are longitudinally stacked or nested within thewaves formed by the longitudinally adjacent winding successively spacedtherefrom.

[0027] As can be seen with respect to FIG. 5, the peaks 31 of the wavesof longitudinally adjacent windings 24 are each linearly aligned so thateach wave is stacked or nested within the next adjacent wave. In optimumconfiguration, the spacing or pitch 26 between each longitudinallysuccessive winding 24 is constructed to be minimal. However, nesting orstacking does occur where the pitch or spacing between longitudinallyadjacent windings 24 is less than 2A i.e. the peak-to-peak amplitude. Aslong as the pitch remains less than 2A each longitudinally adjacentwinding 24 will be nested within the wave formed by the previouslyformed winding 24. By minimizing the pitch or spacing 26 betweenadjacent windings 24, the open space between windings may be minimized.The particular wave-like pattern imparted to wire 22 as shown in FIG. 4allows particularly tight stacking of longitudinally adjacent windings.

[0028] The particular configuration of the stent 20 shown in FIG. 2,provides significant advantages in medical applications. The stent 20 ofthe present invention is typically implanted by means of a ballooncatheter (not shown). The stent 20 in a closed form is held around adeflatable catheter balloon. The stent is then inserted into the lumenand located at the desired position. The shape of the closed stent shownin FIG. 2 permits ease of insertability. As shown in FIG. 3, the stentmay be easily bent or flexed along its longitudinal extent. The spacingor pitch 26 of windings 24 facilitate such bending. This helps in theinsertion and deployment of the stent through a lumen, as typically bodylumens traverse a torturous path through the body which must be followedby the stent which is being deployed therein. Once properly located, theballoon is inflated and the stent is radially expanded for deployment.The balloon is then deflated, and the catheter is removed leaving theexpanded stent in place.

[0029] The windings of stent 20 in closed condition are tightly nested.The cylindrical surface area formed by the coiled wire has greater wiredensity, i.e. more of the surface area is composed of solid wire whileless of the surface area is composed by open space between the wirewindings then in previous non-nested single wire stents. The wiresurface area in the closed condition equals the wire surface area in anexpanded condition. By maximizing the closed condition wire surfacearea, even when the stent is expanded such as shown in FIG. 6, theexpanded wire surface area is also maximized reducing tissue ingrowthbetween the expanded windings of the stent. Contrary to a simple coilspring such as that shown in FIG. 1, the stent 20 of the presentinvention expands without significant foreshortening of the stent orrotation of the ends of the coil. Rather, expansion is achieved by aflattening or elongation of the individual waves of the stent 20. Oncethe stent is expanded after deployment to a shape shown in FIG. 6, theincreased wire surface area as well as the particular shape of the wireprovides sufficient radial strength to resist the compressive forces ofa blocked, constricted or impinged upon lumen.

[0030] Additionally, the above-described benefits of the stent of thepresent invention are achieved without the necessity of longitudinallyoverlapping adjacent wire windings. In many prior art stents, the stentsinclude portions of wire windings which are longitudinally overlapped.This increases the wall thickness of the stent thereat and results in astent which is more difficult to implant in the body lumen by means of aballoon catheter. Also, such stents create an undesirable, moreturbulent fluid flow therethrough. The stent of the present inventionmaximizes wire density, maintains a high degree of flexibility andradial compressive strength without increasing the stent wall thicknessbeyond the single wire diameter.

EXAMPLE

[0031] Mathematically, the geometric analysis of the preferredembodiment of the stent of the present invention may be described asfollows with reference to FIGS. 4 and 5.

[0032] Each wave length 27 of the wave pattern 25 forming stent 20 isformed to include a straight leg segment 29 with a bend radius at peak31. The angle at which the helix coils around the center line χ (FIG. 1)is assumed to be close to 90°, so that the successive windings 24 arepositioned to be as close to concentric as possible while stillmaintaining a helical pattern.

[0033] The integer number of waves N per single circumference or singlewinding follows the equation: ${N = \frac{\pi \quad D}{\lambda}};$

[0034] where D is the diameter of the closed stent and X is the periodof a single wave.

[0035] The number of helical windings M per stent is defined by theequation: ${M = \frac{L\quad \sin \quad \theta}{d_{3}}};$

[0036] where L is the overall stent length; θ is the angle of thestraight leg segments 29 with respect the line of amplitude of the wavepattern; and d₃ is the wire diameter.

[0037] The exterior exposed surface area of the stent is equivalent tothe amount of wire packed within a fixed stent length. The total lengthL_(w) of wire employed to form the stent follows the equation:$L_{w} = {{MN}\left( {{4l} + {4\left( {r\quad + \frac{d_{3}}{2}} \right)\quad \frac{\pi}{180}\left( {90 - \theta} \right)}} \right)}$

[0038] where r is the radius defining the peak curvative; and l is thelength of the straight line segment 29 of the wire.

[0039] It follows that the projected solid wire area is L_(w)d₃ and thepercentage of open space coverage (% open) is given by the equation:${\% \quad {OPEN}} = {100\left( {1 - \frac{L_{w}d_{3}}{\pi \quad {DL}}} \right)}$

[0040] In a specific example, a stent having the parameters listed inTable I and formed in accordance with the present invention yields apercentage of open space (% open) equivalent to 28.959%. TABLE I LLength of Stent 1.000 in D Diameter of Closed Stent 0.157 in d₃ WireDiameter 0.010 in r Radius of Curvative of Peak 0.020 in N Number ofWaves per Winding 3 M Number of Windings per Stent 22.47 l Length ofStraight Portion of Stent 0.097 in

[0041] Further, it is found that an expanded stent constructed inaccordance with the example set forth above, exhibits superiorresistance to pressure P acting upon the stent in a radially compressivemanner (FIG. 6). In the present and illustrative example, P has been hasbeen determined, both mathematically and empirically, to be 10 psi.

[0042] It is further contemplated that the stent-of the presentinvention may be modified in various known manners to provide forincreased strength and support. For example the end of wire 22 may belooped around an adjacent wave or extended to run along the length ofthe stent. The wire may be welded to each winding to add structuralsupport such as is shown in U.S. Pat. No. 5,133,732. Also, each windingsmay be directly welded to the adjacent winding to form a support spinesuch as shown in U.S. Pat. No. 5,019,090.

[0043] Further, as mentioned above, wire 22 is helically wound around amandrel to form the helical pattern shown in FIG. 1. While the angle atwhich the helix coils around the mandrel is quite small, a certain anglemust be imparted to the uniform windings to form a coil. It is furthercontemplated that a helix-like winding may be formed by concentricallywrapping a wave pattern around the mandrel where the length of the sidesof each wave are unequal. As shown in FIG. 7 a wave pattern 125 may beformed having leg segments 129 of uneven length. Wave pattern 125includes individual wave lengths 127 having a first leg segment 129 aand a second leg segment 129 b. Leg segment 129 a is constructed to beshorter than leg segment 129 b. Thus wave pattern 125 has a step-typeshape so that upon winding around a mandrel, the windings 124 coil in ahelical-like fashion therearound. This provides a lengthwise extent tothe coil without having to impart a helical wrap thereto. Forming thestent length in this manner may tend to result in better flowcharacteristics through the stent in use.

[0044] Other modifications which are within the contemplation of thepresent invention may be further described. FIG. 8 shows a wire 222which has been preformed to have a wave pattern 225 which is generallytriangular in shape. This wave pattern 225 includes individual wavelengths 227 having straight leg segments 229 a and 229 b which meet atan apex 231. Wire 222 so formed, may be wound around a mandrel 200. Asthe individual wave lengths 227 nest in a manner above described, theapices 231 of the wave length 227 are longitudinally aligned.

[0045] The winding of wire 222 around mandrel 200 takes place in thefollowing manner. The formed wire Z22 is held in position while themandrel is rotated in the direction of arrow A, thereby coiling the wire222 around mandrel 200. The spacing or pitch 216 is created bysubsequent vertical movement of the of the formed wire 222 along mandrel200 while rotation thereof is taking place. When the winding iscomplete, the ends 233 of the wire 222 may be “tied off” by looping theend 233 around the next longitudinally adjacent winding.

[0046] While in the embodiment shown above, the amplitude of each waveis relatively uniform, it is contemplated that the wire could be formedto have waves of varied amplitude. For example, the wire could be formedso that at the ends of the wound stent the amplitude of the waves isrelatively small while in the central portion of the stent the amplitudeis relatively large. This provides a stent with a more flexible centralsection and more crush-resistant ends.

[0047] In certain situations the stent of the present invention mayinclude a membrane covering (not shown) which would cover the entirestent. The wire surface of the stent would serve as a support surfacefor the membrane covering. The membrane covering would act as a furtherbarrier to tissue ingrowth. Any membrane covering may be employed withthe present invention such as a fabric or elastic film. Further, thismembrane covering may be completely solid or may be porous. In addition,as above described, employing a formed wire having varied amplitudewhere the amplitude of the wire is smaller at the ends of the stentwould help support the membrane covering as the crush-resistant endswould serve as anchors to support the membrane covering with littlesupport necessary at the more flexible central section of the stent.

[0048] Various changes to the foregoing described and shown structureswould not be evident to those skilled in the art. Accordingly, theparticularly disclosed scope of the invention is set forth in thefollowing claims.

What is claimed is:
 1. An intraluminal stent comprising: a generallyelongate tubular body formed of an elongate helically wound wire, thewire being formed into successive waves along the length of the wire,the waves being arranged in non-overlapping longitudinally spacedsuccession along the length of said tube, the longitudinal spacing ofthe helical windings being less than twice the amplitude of the wave. 2.An intraluminal stent of claim 1 wherein longitudinally adjacent ones ofsaid waves are longitudinally nested along the length of said tubularbody.
 3. An intraluminal stent of claim 2 wherein said longitudinallynested waves define peaks which are linerally aligned.
 4. Anintraluminal stent of claim 1 wherein said longitudinal spacing of thehelical windings is less than the amplitude of the wave.
 5. Anintraluminal stent of claim 1 wherein said stent includes said wirebeing helically wound in non-overlapping disposition and wherein saidwire defines an open area between said helically wound wire and whereinsaid percentage of open surface area of said stent in relationship tothe total surface area of said stent is less than 30% in the closedcondition.
 6. An intraluminal stent of claim 1 wherein said tubular bodyis uniformly flexible along the length thereof.
 7. An intraluminal stentof claim 6 wherein said stent is radially expandable after intraluminalimplantation.
 8. A radially expandable generally tubular endoluminalimplantable prosthesis comprising: a wire which is wound in a helicalconfiguration to define a generally elongate tubular body, the wireincluding successively formed waves along the length of said wire, eachwire wave being non-overlappingly nested within the wave formedlongitudinally thereadjacent.
 9. A prosthesis of claim 8 wherein saidwire waves are of generally uniform configuration defining apeak-to-peak amplitude of a preselected first dimension.
 10. Aprosthesis of claim 9 wherein said longitudinally adjacent wire wavesare spaced apart a preselected second dimension which is less than thepreselected first dimension.
 11. A prosthesis of claim 10 wherein saidwire has a given wire diameter and wherein said wound wire defines agenerally cylindrical outer surface having solid portions formed by saidwire and open portions formed between said wound wire.
 12. A prosthesisof claim 11 wherein said generally cylindrical outer surface defines atotal surface area including an open surface and a wire surface andwherein said non-expanded wire surface substantially exceeds said opensurface.
 13. A prosthesis of claim 12 wherein said open surface area isless than 30% of said total surface area.
 14. An intraluminal stentcomprising: an elongate tubular body formed of a single wound wire; saidwire having a wave-like pattern defining a plurality of waves formedalong the length of said wire, each said wave defining a leg segmentbetween wave peaks, each leg segment being of a length different fromthe next adjacent leg segment.
 15. An intraluminal stent of claim 14wherein said wire is wound about a central axis forming said tubularbody.
 16. An intraluminal stent of claim 15 wherein tubular bodyincludes longitudinally successive waves along the length thereof, eachsaid wave being nested within the wave formed longitudinallythereadjacent.
 17. An intraluminal stent of claim 14 wherein each waveis defined by a peak and a pair of wave leg segments extending from saidpeak.
 18. An intraluminal stent of claim 17 wherein one of said wave legsegments of said pair has a length greater than the other wave legsegment of said pair.